The present invention relates to a brushless DC motor having a permanent magnet at a rotor thereof. Further in details, the invention relates to a brushless DC motor utilizing a reluctance torque for effectively utilizing the reluctance torque while preventing a cogging torque from increasing as less as possible.
Conventionally, there is a brushless DC motor of this kind utilizing a reluctance torque having a constitution shown by, for example, FIG. 47. That is, according to a rotor 101 of a brushless DC motor of the drawing, a permanent magnet 102 is arranged on an inner side of an outer periphery to some degree. Therefore, a magnetic region 103 is present between the outer periphery of the rotor 101 and the permanent magnet 102. In driving the motor, as shown by an arrow mark of “q axis” in the drawing, a magnetic flux produced by a stator passes through the magnetic region 103 of the rotor 101. The magnetic flux generates a reluctance torque. Meanwhile, a magnetic flux produced by the permanent magnet 102 also passes through the magnetic region 103 (“d axis” in the drawing). The magnetic flux generates a magnet torque. Further, in the drawing a stator coil and a lower half of the motor are omitted (similar to other drawing of the same kind).
However, according to the above-described conventional brushless DC motor utilizing the reluctance torque, there poses a problem explained below. That is, according to the brushless DC motor, as mentioned above, a magnetic path is commonly used by the magnetic flux produced by the permanent magnet 102 and the magnetic flux produced by the stator (magnetic region 103). The following unpreferable various phenomena are brought about thereby. Further, the phenomena are similar also in a brushless DC motor of a type providing a plurality of rows of permanent magnets at respective magnetic poles of a rotor as shown by FIG. 48.
First, it is pointed out that the magnetic region 103 is significantly magnetized. Therefore, the magnetic region 103 is liable to fall into magnetic saturation. When the magnetic saturation is brought about, the torque of the motor is not proportional to magnetic excitation of a stator coil and is not increased considerably. Therefore, the energy efficiency is poor. Therefore, a characteristic of the motor is deteriorated and vibration or noise by harmonics are increased by that amount.
Further, since the magnetic path is commonly used by the two magnetic fluxes, when some measure is taken with regard to one of the magnetic torque and the reluctance torque, an influence is effected on the other thereby. For example, when there is taken a measure of reducing a cogging torque component of the magnet torque, a magnitude of the reluctance torque or a torque waveform is influenced thereby. Or, when the reluctance torque is intended to be increased, a magnitude of the magnet torque or the torque waveform is conversely influenced thereby. Therefore, design of the motor is very difficult.
Further, there also poses a problem that a magnetic flux density distribution becomes uniform at a gap between the rotor and the stator. An explanation will be given of the problem in reference to a graph of FIG. 49. The abscissa of the graph designates an angle around an axis of the brushless DC motor of FIG. 47. A section A of the angle designates an angular range in correspondence with the single permanent magnet 102 in the rotor 101. Meanwhile, the ordinate designates a magnetic flux density. Further, a broken line designates a magnetic flux density distribution in no load time (when the stator is not excited). A bold line designates a magnetic flux density distribution in load time (when the stator is excited). According to the graph, in no load time (broken line), the magnetic flux distribution is substantially uniform in the section A. In contrast thereto, in load time (bold line), the magnetic flux distribution is deviated even in the section A. The deviation is caused by influence of the magnetic flux of the q axis. That is, because even in the angular range in correspondence with the single permanent magnet 102, at a right half thereof and a left half thereof, at one of them, the magnetic flux is strengthened and at other thereof, the magnetic flux is weakened. The more strengthened the excitation current of the stator in order to achieve a strong torque, the more significant is a warp of the magnetic flux distribution.
Further, according to the brushless DC motor of FIG. 47, a leakage magnetic flux flowing from one permanent magnet to a contiguous permanent magnet without interlinking with the stator coil, is so large that the leakage magnetic flux cannot be disregarded. This signifies that magnetic force of the permanent magnet cannot be made full use as the magnet torque.
According to a conventional brushless DC motor, a torque function is promoted by increasing an amount of using magnets in a rotor as large as possible. For example, according to a rotor shown in FIG. 50, an effective magnetic pole opening angle of a magnet is set to be near to almost 180° in electric angle. Thereby, the magnetic flux is increased and a torque function is ensured by a magnet torque.
However, according to the above-described conventional technology, there poses the following problem. That is, the higher the torque of the brushless DC motor is intended to achieve, the larger the number of magnets are needed. The fact constitutes a factor of cost. Further, when the motor is of a type in which a magnetic member of a rotor is present on an outer side of a magnet as shown by FIG. 50 for utilizing a reluctance torque, there also poses the following problem. That is, a magnetic path is commonly used by a magnetic flux related to a magnet torque and a magnetic flux related to a reluctance torque. Therefore, the magnetic member at the commonly used portion is liable to fall to magnetic saturation. Due to the fact, there is a case in which a torque in proportion to excitation is not achieved. Further, a magnetic flux distribution in an air gap between a rotor and a stator, becomes significantly nonuniform due to the magnetic flux related to the reluctance torque. The fact deteriorates properties of the motor and causes vibration or noise by harmonics.